It has to be well timed. It needs to have the right components that maybe contain emerging technologies or something like, say, when Doom came out — the Network play — there weren't many games like that. There was a really great 3D world that a lot of people hadn't seen. It was light-years ahead of Wolfenstein. It was shareware, so it had Internet distribution. We used the Internet to get it all over the place. So it used a lot of stuff that was just becoming popular at that time. id just capitalized on it.

In response to the question, "What makes a classic game?" (quoted from Romero's own webpage)